18446744073709551615

18446744073709551615 is the decimal representation of the maximum value of an unsigned 64-bit integer. It equals 2^64 − 1, or 18,446,744,073,709,551,615 in decimal form. In hexadecimal, this value is 0xFFFFFFFFFFFFFFFF, and in binary it is represented by 64 consecutive 1 bits.

This number is a fundamental constant in computer science for architectures and languages that use 64-bit unsigned integers. It is the largest value that can be stored in an unsigned 64-bit variable in many programming languages, including C, C++, Rust, Go, and others. By contrast, the maximum value for a signed 64-bit integer is 2^63 − 1 (9,223,372,036,854,775,807), which means 18446744073709551615 does not fit into a signed 64-bit type.

Common uses of this value include defining upper bounds for data structures, serving as an all-ones bitmask, and acting as a modulus in 64-bit arithmetic. In some APIs and libraries, it is exposed as a named constant such as UINT64_MAX or UInt64.MaxValue, guiding developers in writing portable and well-defined code. The constant also illustrates the practical limits of fixed-width integer representations in modern computing.